In maternofetal transfer of passive immunity, IgG is transferred across cells of fetal membranous tissue, the yolk sac membrane (YSM) in the case of the rabbit. The binding of IgG to Fc Receptors of the YSM is the first step in this process and the one which determines its overall specificity. Receptor-bound IgG is presumed to undergo lateral migration in the plane of the membrane producing clusters of Receptor-IgG complexes that are pinocytosed and subsequently transferred to the fetus. Our long-term objective is to characterize the structure and function of the Receptor. To date, we have succeeded in solubilizing an IgG-binding receptor fraction from the YSM and in isolating the active portion by gel filtration. We propose to complete the purification and chacterization of Receptor and subsequently, by restoring it to a natural or synthetic (liposome) membrane environment, to study the clustering of Receptor under the influence of ligand, IgG.